Revert "rtos/FreeRTOS: Fix FreeRTOS thread list reading"

Revert "rtos/FreeRTOS: Fix FreeRTOS thread list reading"

This reverts commit 6568d29cc1d0d94daafec5bdb73de7d4f17da257.
The reverted change caused some tasks were missing in thread list.

While on it add a comment explaining the relation of uxTopUsedPriority
and configMAX_PRIORITIES, introduce config_max_priorities and
change types to unsigned.

Change-Id: I4371c8882470d13ee7360ef21b132c56ecb95af8
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/5577
Tested-by: jenkins